Among the other uncataloged items at the Washington, D.C., library was a letter, written in 1929 by Kilmer’s widow, Aline, that one local researcher says solves the decades-old literary mystery of where the poem was penned — the couple’s former home in Mahwah.

“This is a tremendous find,” said Alex Michelini of Mahwah, a retired journalist and founder of the Joyce Kilmer Society of Mahwah, who discovered the items this week in his three-year quest to locate the lost notebook.

Over the years, several locales have tried to stake a claim on where the poem was penned or inspired — including New Brunswick; Swanzey, N.H.; Montague City, Mass.; Darien, Conn.; and the University of Notre Dame. In February, to mark the 100th anniversary of the poem, Mahwah officials issued a proclamation declaring their township to be the birthplace of “Trees.”

The date of the poem, Feb. 2, 1913, has always pointed to a time when Kilmer lived in Mahwah. But the beyond-a-doubt proof that he was in the township when he wrote the poem has been missing – until this week, Michelini said Thursday.

The worn leather-bound notebook and Aline Kilmer’s letter were among about a dozen boxes donated to the university by Miriam A. Kilmer, the poet’s granddaughter, in December 2004 upon the death of her parents, Kenton and Frances.

Michelini, who had been searching for the notebook — referred to in interviews by Kenton, who died in 1995, and the poet’s daughter Deborah, a Roman Catholic nun who died in 1999 — said he was rifling through several of the cataloged boxes earlier this week when a library staffer brought him another set of boxes not listed in the public record.

That’s where he found Aline’s letter to a graduate student stating the circumstances of the poem’s origin. But the notebook was not with it, he said.

“I kept asking, ‘What about the notebook?’Ÿ” he said, adding that he was going to leave but they told him to come back the next day. “It was in a secret place where the manuscript manager felt it would be safer. That’s their prize.”

Miriam Kilmer said in a phone interview from her Virginia home Thursday that she donated the boxes to Georgetown after her mother died, to carry out her father’s wishes. It was Kenton Kilmer’s alma mater and has a collection of rare manuscripts.

Kilmer’s granddaughter said library personnel were aware when she donated the boxes that the notebook containing “Trees” was in there.

But the people she dealt with are now retired, she said. Michelini also said the library told him the manuscript manager who hid the notebook had retired.

“I’m not terribly happy they lost track of the items — particularly of the notebook,” Miriam Kilmer said. “But anyway, it’s been found now. That’s the important thing.”

Calls to Georgetown University were not returned Thursday.

The 6-inch notebook containing about 190 numbered pages has two entries with the poem, Michelini said. On one page is the title and famous first two lines of the poem and the date. Later in the book the entire poem is scribed, likely in Aline’s handwriting, Michelini said, adding that family members have said the husband and wife, also a poet, often took dictation for each other.

But it’s the letter Aline Kilmer wrote on March 25, 1929 — 11 years after her husband died ¬fighting in World War I — to a graduate student that Michelini says is the definitive proof. The document contains the address in the Coytesville section of Fort Lee, where Miriam Kilmer lived at |the time, and reads:

“The poem, I definitely remember, was written at home, in the afternoon, in the intervals of some writings. The desk was in an upstairs room, by a window looking down a wooded hill.”

The site to which she refers is the Ramapo Valley as seen from the couple’s Airmont Road home.